How can I upload Blender videos to Youtube?
Blender files are not video files. You need to render and export your video sequence in order to get a playable video.

(This is my workflow for Youtube publishing. Based on the "Advanced encoding settings" article and Blender's limitations. Of course, there are plenty of other ways to do it, but this is how I do it.)

Be advised that there are issues with Blender's output interface since 2.71. This is planed to be fixed around 2.74.

Video settings


Open the properties window and open the render context with the triangle button.
Under the Dimensions panel, select an appropriate resolution and frame rate. The resolutions that Youtube uses are the following:

2160p: 3840x2160
1440p: 2560x1440
1080p: 1920x1080
720p: 1280x720
480p: 854x480
360p: 640x360
240p: 426x240

All of these use the 16:9 aspect ratio. So if you're using a different aspect ratio, such as 4:3, youtube will covert it to 16:9 by adding black empty space on the left and right sides. Use one of the above resolutions for predictable results on Youtube. Note that this is not the same thing as the aspect ratio shown in Blender!
Make sure that you have set the end frame for however long you want your video to be. This can be calculated by frame rate x length of video in seconds.
Open the Output panel. Select H.264 for the file format. When you select that, a new panel named Encoding should appear.
Open the Encoding panel. Select the Quicktime format.
Select the H.264 codec.
Set the max bit rate to 0 to make Blender ignore it. 
Set the bit rate to one of the following depending on your video resolution (These are only suggestions given by Youtube themselves. You can find higher quality suggestions on this page.)

2160p: 35000-45000 (kb/s)
1440p: 10000 (kb/s)
1080p: 8000 (kb/s)
720p: 5000 (kb/s)
480p: 2500 (kb/s)
360p: 1250 (kb/s)


Audio settings

If you want audio in your video:


Select AAC as your audio codec under the Encoding panel
Use whatever bitrate your source audio was recorded in.


Render and export

In order to finally render and export your video:


First, make sure you save your blend file somewhere. I've had issues of it not wanting to export if hadn't saved the blend first.
Under the Output panel, set where you want to export your video.
Go to the Render menu and select render animation.


You will get a .mov file in the folder where you exported your video. You can upload this to Youtube.

If you want to be finicky, you can convert that Quicktime file to MP4 using ffmpeg. 

ffmpeg -i INPUT.mov -vcodec copy -acodec copy OUTPUT.mp4